July 15, 2004
10:00 AM (EDT)

News Release Number: STScI-2004-24

Astronomers Measure Mass of a Single Star  First Since the Sun

July 15, 2004: Astronomers have directly measured the mass of a single star 
the first time such a feat has been accomplished for any solitary
star other than our own Sun. The measurement has been done on a
small red star located some 1,800 light-years from Earth. Knowing
the masses of stars is important in understanding stellar evolution.

Q & A: Understanding the Discovery

1.
Why do astronomers search for gravitational microlensing events?

Astronomers scan for microlensing events to look for evidence of
dark matter in the outer fringes, or halo, of our galaxy. Believed
to make up a significant chunk of our universe, dark matter may be
partly made up of celestial objects that are too faint to see because
they are very cool. These objects, referred to as "Massive Compact
Halo Objects," or MACHOs, may be lurking in the halos of galaxies.
They may include black holes, compact stars, and failed stars called
brown dwarfs. If some dark matter were in the form of MACHOs, then
its presence could be detected by the gravitational influence MACHOs
would have on light from distant stars. If a MACHO object passes in
front of a star in a nearby galaxy, such as the Large Magellanic Cloud,
then the gravitational field of the MACHO will act like a lens, causing
the brightness of the background star to increase as the MACHO passes by.

Astronomers also have discovered that gravitational microlensing events
can yield other important information about a star, including whether the
"lensed" star has planets circling it. Microlensing also provides
information on background stars that are normally too dim and too far
away to be seen with telescopes.

2.
Why have astronomers not been able to measure the mass of other solitary stars, aside from the Sun?

Until now, the only way astronomers have successfully measured the mass
of a star is by studying its gravitational effect on another star. Both
stars are bound together by gravity in a binary-star system.

With the gravitational microlensing technique, astronomers determine the
mass of a star by measuring its effect on beams of light passing by the
star. Microlensing events, however, occur in about one in 10 million stars.
So, determining the mass of a star with this method is difficult because
astronomers must use a powerful computer to scan millions of stars to find
a microlensing event. Astronomers also must be able to image both stars in
order to calculate the distance to them, or they must look for subtle
changes due to the motion of the Earth around the Sun during the
microlensing event. The distances are needed to determine the mass of the
foreground star.